In a number of different surgical procedures it is necessary to make cuts in an eye. To name just two examples, cuts are made in corneal tissue as part of a so-called LASIK surgery (LASIK: laser in-situ keratomileusis) in preparation for a corneal flap, as it is often referred to in English terminology, or as part of an intracorneal lenticule extraction for separation of a volume of corneal tissue, which is then removed from the cornea. Laser equipment has recently been developed to make it possible to create such cuts using laser technology.
The physical effect of the so-called laser-induced optical breakdown is utilized in creating cuts by means of focused laser radiation in transparent or translucent material (transparent/translucent for the laser radiation). The penetration leads to a photodisruption of the irradiated tissue in the region of the focus of the laser radiation. The interaction of the incident laser radiation with the irradiated corneal tissue causes a local evaporation of tissue at the focal point. To ensure a reproducible, high-quality cut, the properties of the laser must be checked at sufficiently short intervals of time and readjusted, if necessary. An important factor here is the pulse energy of the pulsed cutting laser being used. To ensure a high-quality cut, the energy actually incident on the surface of the cut must be known and adjustable accordingly. If the pulse energy is too low, it may result in the cut being too deep and/or not efficient enough. If the pulse energy is too high, it may result in the cut not being deep enough and/or too wide and may thus lead to unwanted damage to neighboring tissue. Over a period of time, the pulse energy that is actually incident upon a cut surface may deviate from the pulse energy set on the cutting laser. In the prior art, a calibration is therefore performed at regular intervals, so that an energy measuring device (for example, a conventional power meter) can be used to determine the power and/or pulsed energy actually incident upon the cutting surface and to adjust the pulse energy set on the cutting laser accordingly.
However, it has been found that the result of the photodisruption (for example, the size of the gas bubbles produced) does not depend only on the amount of incident energy but is also influenced by other factors such as, for example, the beam diameter, the pulse length, the energy control of the device, etc. The cutting result to be expected may vary if one of the aforementioned factors changes during production (e.g., due to production spread) or during use of the cutting laser.
The final cutting result is achieved by a combination of incident laser light dose and placement of pulses which are arranged in a spiral or linear arrangement. The cutting effect and the respective settings for use on human corneal tissue depend to a high degree on each of the aforementioned factors. A change in any of the factors will change the cutting process per se. The dose is usually adjusted by varying the energy and if this is not sufficient the cutting pattern is rearranged in a narrower or more controlled manner. Nevertheless the total dose of the incident laser beams may have serious effects on the quality of the flap or may cause corneal reactions such as inflammation, DLK, haze, etc.